Progressive cavity pumps (PCP) are well known for production of fluids from a wellbore. PCP's comprise a stator and a rotor. The stator is a tubular element having an elastomeric helical passage formed therethrough. The stator is installed in the production tubing which is then lowered into the wellbore. The rotor is a helical element that rotates in the helical passage to pump wellbore fluids uphole through the stator, and through the production string to surface. The rotor is installed in the stator and is suspended from, and rotatably driven, by a sucker rod string which extends downhole through the wellbore from surface.
It is well known that placement of the rotor in the stator is inexact, load causing axial displacement of the rod string and rotor suspended therefrom. One factor includes pumping torque loads on the rod string which vary dramatically during the pumping operation due to differences in gradients of fluids being pumped and the passage of solids through the pump. When the torque increases on the rod string, the consequent torsional deformation of the rod string produces a shortening of the rod string. Conversely, when torque on the rod string decreases, the rod string lengthens. Further, uphole pumping loads impose downhole axial loads on the rod string. The lengthening and shortening of the rod string causes the rotor to move vertically up and down inside the stator which detracts significantly from pump efficiency and can lead to premature wear of the pump.
A further complicating factor in dealing with rotor placement is the fact that the movement of the rod string to rotor connection is eccentric, rotating not only about a rotor axis but also moving in a circular or elliptical path. The eccentric motion of the rotor causes the rod string to move violently laterally within the production string, leading to premature wear of the production string and undesirable stresses on both the production string and the rod string. The lateral motion may also cause the pump stator to move from side to side in the production casing, which is also undesirable and may result in wear of the production string.
Others have attempted to reduce movement of the rotor and to support the rod string from the radial and axial loading imposed thereon. U.S. Pat. No. 5,209,294 to Weber teaches sealed bearings which act between the rod string and the production tubing to transfer loads to the production tubing. Bypass passageways are provided to direct wellbore fluids away from the sealed bearings so as to increase bearing life.
Clearly there is a need for efficient means to support the rod string under axial and radial loading on the rod string and the eccentric motion of the rotor and connected rod string.